Method and equipment for measuring vapor flux from surfaces

ABSTRACT

A method and equipment for measuring vapor flux from a surface e.g. the rate of water loss from human skin which is useful in the evaluation of the efficiency of the skin/water barrier uses a closed cylinder which is placed with the open end against the skin and the closed end is cooled. By measuring the temperature and relative humidity within the cylinder the water vapor flux can be determined.

The present invention relates to a method and a device for measuringvapour flux from a surface, more particularly it relates to a method anda device which can be used to measure the rate of water loss from humanskin.

The transepidermal water loss (TEWL) is important in the evaluation ofthe efficiency of the skin/water barrier. Damage to the skin resultingfrom various skin diseases, burns and other damage can affect the TEWLand measurement of the TEWL can indicate such damage and possibly itsearly onset or response to treatment. It therefore has use in clinicaldiagnosis.

As the TEWL is a measure of the effectiveness of the skin/water barrierits measurement is important in prematurely born infants, whendehydration due to excessive water loss can have serious adverseresults. The TEWL, is also used in testing the effect of pharmaceuticaland cosmetic products applied to the skin.

GB patent 1532419 describes an instrument for measuring the rate ofwater loss from the skin in which an open cylinder containing two spacedapart relative humidity sensors and two temperature sensors is placed onthe skin so that water vapour escaping from the skin diffuses along thecylinder and passes the sensors. The output from these sensors can beused to measure the concentration gradient of water vapour in thecylinder and hence the water vapour flux from the skin.

However this instrument can only function accurately in a homogeneousdiffusion zone which means that the air inside the cylinder must bestill. In practice, air currents and other air movements in the vicinityof the open end of the cylinder affect the measurements and introduceerrors. Other errors are associated with changes of ambient humidity.

We have now devised equipment and a method for measuring the watervapour flux from a surface which reduces these problems.

According to the invention there is provided equipment for measuring thewater vapour flux from a surface which comprises a cylinder with anopening at one end, which opening is adapted to be placed against thesurface of interest, the other end of the cylinder being closed by meansof a cooled surface; the cylinder containing sensors, which sensorsbeing able to measure quantities, such as relative humidity andtemperature, from which the flux of water vapour from the surface ofinterest can be calculated.

The invention also provides a method for measuring the water vapour fluxfrom a surface, which method comprises enclosing a zone adjacent to thesurface of interest within a cylinder which is open at one end andclosed at the other end, by placing the open end of the cylinder againstthe surface of interest, cooling the surface closing the closed end ofthe cylinder and measuring quantities, such as relative humidity andtemperature within the cylinder, from which the flux of water vapourforms the surface of interest can be calculated.

The closed surface is preferably cooled to a temperature at which thewater vapour in its vicinity condenses to liquid water or ice and steadyconditions of water vapour diffusion are established within the cylinderwith the concentration of water vapour in the immediate vicinity of thecold surface being lower than in the immediate vicinity of the surfaceof interest. By this means, a water vapour concentration gradient isestablished alone the length of the cylinder.

Given a suitable geometrical arrangement of sensors, their readings oftypically relative humidity and temperature can be used to calculate thewater vapour concentration gradient along tete length of the cylinder.The relationship between the water vapour flux and the water vapourconcentration gradient along the length of the cylinder is well known,for example it is explained in GB 1532419.

Various methods of measuring the water vapour concentration gradientalong the length of the cylinder exist. In one implementation, theconcentration of water vapour at a known distance from the cooledsurface is measured and, using the known concentration of water vapourin the immediate vicinity of the cooled surface, the gradient of watervapour concentration along the length of the cylinder can thecalculated.

The concentration of water vapour can be measured by measuring therelative humidity, in which case there is preferably a second sensorwhich is able to measure the temperature substantially at the locationof the sensor which measures the relative humidity. From measurements ofthe relative humidity and associated temperature, the concentration ofwater vapour at the sensor location can be determined.

A suitable and convenient choice of relative humidity sensor includessensors based on the change in capacitance or change in electricalconductivity etc., which are widely commercially available. A suitableand convenient choice Of temperature sensor includes the conventionalthermocouple and thermistor, which are widely commercially available.Alternatively a composite sensor can be used which simultaneouslymeasures the relative humidity and the temperature so that one suchcomposite sensor can produce the required signals.

Preferably the sensors take their measurements at a point or in a planesubstantially parallel to the surface of interest, in order to make themeasurement of water vapour flux more accurate.

An additional temperature sensor is preferably placed in contact withtile cold surface in order both to maintain its temperature at aconstant value and to provide a temperature reading from which theconcentration of water vapour in its immediate vicinity can becalculated.

Preferably the outputs from the sensors are fed to a processing devicesuch as a microprocessor or PC, which is programmed to convert thesignals from the sensors to the required type of output or readout. Bythis means a user of the equipment can obtain a result in a form whichrequires little further processing and can be interpreted easily e.g.the flux of water vapour can be directly displayed.

The closed surface of the cylinder can be cooled by conventional coolingmeans and preferably uses an electrical cooling means e.g. one based onthe Peltier effect. This enables the cooling to be accurately controlledat the requisite level quickly and easily.

The water which is condensed at the cold surface can be re-evaporated byheating the surface during times when the instrument is not being usedfor measurement. If tile cooler is a Peltier device, then this canconveniently be accomplished by reversing the current flow through it.

The cylinder is the common geometry of measurement chamber for suchinstruments, but any convenient shape can be used e.g. rectangularparallelepiped, prism, etc. The measurement chamber is preferably madeof compact size so that it occupies a small area and can easily beplaced on the surface of interest, e.g. the skin at the desired locationof a TEWL measurement. The measurement chamber can conveniently beconstructed in the form of a wand or with a convenient handle etc.

The equipment and method can be used to measure any vapour flux from asurface although, when the vapour is not water vapour, the sensor andcold plate temperature are chosen accordingly.

The equipment can be used with any surface and apart from skin theequipment can be used to measure water vapour flux from plants, etc.

In use, the open end of the cylinder is placed against the skin and theclosed end of the cylinder is cooled and readings from the sensors arefed to a processor which converts these readings to the desired waterflux measure. After a short period to allow the measurement conditionsto stabilise, readings are taken. Alternatively, readiness can be takencontinuously or periodically in order to record the time change of thesignals and the water vapour flux calculated according to suitablecriteria.

Unlike existing equipment, which is open to the atmosphere to allow thewater vapour flux to escape, the closed cylinder means that neither airmovement nor humidity in the air outside the cylinder can affect themeasurements taken and so more accurate readings are obtained.

In the implementation described, only one relative humidity sensor andtwo temperature sensors are required, thus simplifying the construction.This does not preclude the use of more sensors, however. If relativehumidity and associated temperature are sensed at two locations withinthe cylinder, as with conventional embodiments of such instruments, thenthe gradient of water vapour concentration can be calculated withoutrequiring a knowledge of the water vapour concentration in the immediatevicinity of the cold surface. The use of additional sensors would allowmore precise calculations of water vapour flux to be performed. It mayalso be convenient to incorporate additional sensors in the equipment,e.g. to measure ambient temperature, skin temperature, etc.

An alternative means of measuring water vapour flux is to measure themass of water condensed on the cold surface of the cylinder, the mass ofwater per unit interval of time being numerically equal to the flux ofwater vapour emanating from the surface of interest. A convenient masssensor, such as a quartz microbalance, can therefore be used in place ofthe relative humidity sensor(s) in an alternative implementation of thedevice.

An alternative means of measuring water vapour concentration in thecylinder includes a sensor based on measuring the absorption of infraredradiation of suitable wavelength by the water vapour.

The invention is described with reference to the accompanying drawingwhich is a side view of an embodiment of equipment according to theinvention.

In the drawing a chamber in the from of cylinder (1) is open at end (1a) and is closed at the end (1 b) by a surface (2) which is in contactwith a Peltier cooling device (3). Inside the cylinder (1) is acapacitative relative humidity sensor (4) and a thermocouple (5) whichmeasures the temperature at the location of the relative humidify sensor(4). An additional thermocouple (7) is placed in contact with the cooledsurface (2). The outputs of (4), (5) and (7) are fed to a computer (notshown).

To measure the water vapour flux from the skin (6) of a person, the openend (1 a) is placed against the skin as shown and the surface (2) iscooled down to a sufficiently low temperature to maintain asubstantially lower water vapour pressure in its immediate vicinity thanin the immediate vicinity of skin at the other end of the cylinder.

The computer is programmed with a program so that the output from thesensors (4), (5) and (7) are converted to a reading in the desired form,e.g. water vapour flux from the surface.

After a short period of time (to allow for steady state conditions to heattained inside cylinder (1)), the readings are evaluated by thecomputer. Alternatively, readings can be taken continuously orperiodically in order to record the time change of the signals and thewater vapour flux calculated according to suitable criteria.

After the measurement or periodically, when convenient, the cold surfaceis heated to re-evaporate condensate, thus preventing a build-up ofcondensate (liquid water or ice).

What is claimed is:
 1. Equipment for measuring a water vapour flux froma surface which comprises a cylinder with a first end which is open anda second end which is closed, the first end being adapted to be placedagainst said surface and there being a cooling means adapted to cool thesecond end of said cylinder to form a cold cylinder end; the cylindercontaining one or more sensors which are able to measure a plurality ofquantities from which the flux of water vapour from said surface can becalculated.
 2. Equipment as charmed in claim 1 in which the said sensorsare able to measure relative humidity and temperature.
 3. Equipment asclaimed in claim 2 in which there is first sensor able to measure therelative humidity and a second sensor which is able to measure thetemperature substantially at the location of the first sensor. 4.Equipment as claimed in claim 2 in which the sensor for measuringrelative humidity is based on the charge in capacitance or change inelectrical conductivity.
 5. Equipment as claimed in claim 2 in which thesensor for measuring relative humidity is based on the change incapacitance or change in electrical conductivity.
 6. Equipment asclaimed in claim 1 in which said sensor(s) comprise means for measuringthe mass of water condensed on the cold end of the cylinder. 7.Equipment as claimed in claim 1 in which the sensor(s) comprise a quartzmicrobalance.
 8. Equipment as claimed in claim 1 in which the sensorwhich is able to measure quantities from which the flux of water vapourfrom the surface can be calculated is a sensor based on measuring theabsorption of infrared radiation of suitable wavelength by the watervapour.
 9. Equipment as claimed in claim 2 in which the sensor formeasuring the temperature is a thermocouple or a thermistor. 10.Equipment as claimed in claim 3 in which the sensor for measuring thetemperature is a thermocouple or a thermistor.
 11. Equipment as claimedin claim 6 in which the sensor for measuring the temperature is athermocouple or a thermistor.
 12. Equipment as claimed in claim 8 inwhich the sensor for measuring the temperature is a thermocouple or athermistor.
 13. Equipment as claimed in claim 1 in which the sensorwhich is able to measure quantities from which the flux of water vapourfrom the surface can be calculated is a composite sensor whichsimultaneously measures the relative humidity and the temperature. 14.Equipment as claimed in claim 1 in which the sensor or sensors arepositioned so as to take their measurements at a point or in a planesubstantially parallel to the surface.
 15. Equipment as claimed in claim2 in which the sensor or sensors are positioned so as to take theirmeasurements at a point or in a plane substantially parallel to thesurface.
 16. Equipment as claimed in claim 8 in which the sensor orsensors are positioned so as to take their measurements at a point or ina plane substantially parallel to the surface.
 17. Equipment as claimedin claim 1 in which there is an additional temperature sensor is adaptedto be placed in contact with the second end of the cylinder. 18.Equipment as claimed in claim 8 in which the outputs from the sensorsare fed to a processing device.
 19. Equipment as claimed in claim 13 inwhich the cooling means is based on the Peitier effect.
 20. Equipment asclaimed in claim 6 in which the cooling means is based on the Peltiereffect.
 21. Equipment as claimed in claim 8 in which the cooling meansis based on the Peltier effect.
 22. Equipment as claimed in claim 14 inwhich the cooling means is based on the Peltier effect.
 23. Equipment asclaimed in claim 17 in which the cooling means is based on the Peltiereffect.
 24. A method for measuring a water vapour flux from a surface,which method comprises enclosing a zone adjacent to the surface within acylinder which at one end and closed at the other end, by placing theopen end of the cylinder against the surface, cooling the closed end ofthe cylinder to form a cold end and measuring a plurality of quantitiesfrom which the flux of water vapour from the surface of interest can becalculated.
 25. A method as claimed in claim 24 in which the closed endof the cylinder is cooled to a temperature at which the water vapour inits vicinity condenses to liquid water or ice and steady conditions ofwater vapour diffusion are established within the cylinder, with theconcentration of water vapour in the immediate vicinity of the cold endof the cylinder being lower than in the immediate vicinity of thesurface.
 26. A method as claimed in claim 24 in which the concentrationof water vapour at a known distance from the cooled end of the cylinderis measured and, using the known concentration of water vapour in theimmediate vicinity of the cooled end, the gradient of water vapourconcentration along the length of the cylinder calculated.
 27. A methodas claimed in claim 25 in which the concentration of water vapour at aknown distance from the cooled end of the cylinder is measured and,using the known concentration of water vapour in the immediate vicinityof the cooled end, the gradient of water vapour concentration along thelength of the cylinder calculated.
 28. A method as claimed in claim 24in which the concentration of water vapour is measured by measuring therelative humidity and the temperature at simultaneously at the samelocation.
 29. A method as claimed in claim 25 in which the concentrationof water vapour is measured by measuring the relative humidity and thetemperature at simultaneously at the same location.
 30. A method asclaimed in claim 26 in which the concentration of water vapour ismeasured by measuring the relative humidity and the temperature atsimultaneously at the same location.
 31. A method as claimed in claim 24in which the relative humidity is measured by a sensor which measuresthe change in capacitance or change in electrical conductivity.
 32. Amethod as claimed in claim 24 in which the temperature is measured bymeans of thermocouple and thermistor.
 33. A method as claimed in claim24 in which the sensor or sensors take their measurements at a point orin a plane substantially parallel to the surface.
 34. A method asclaimed in claim 24 in which the concentration of water vapour ismeasured by measuring the relative humidity and the temperature atsimultaneously at the same location.
 35. A method as claimed in claim 24in which the concentration of water vapour is measured by measuring therelative humidity and the temperature at simultaneously the samelocation.
 36. A method as claimed in claim 24 in which there is atemperature control means placed in contact with the cold end of thecylinder to maintain its temperature at a constant value and from whichtemperature the concentration of water vapour in its immediate vicinityis calculated.
 37. A method as claimed in claim 24 in which the outputsfrom the sensors are fed to a processing device which is programmed toconvert the signals from the sensors to the required type of output orreadout.
 38. A method as claimed in claim 24 in which the outputs fromthe sensors are fed to a processing device which is programmed toconvert the signals from the sensors to the required type of output orreadout.
 39. A method as claimed in claim 33 in which the outputs fromthe sensors are fed to a processing device which is programmed toconvert the signals from the sensors to the required type of output orreadout.
 40. A method as claimed in claim 24 in which the closed end ofthe cylinder is cooled by a cooling means based on the Peltier effect.41. A method as claimed in claim 24 in which the water which iscondensed at the cold end of the cylinder is re-evaporated by heatingsaid cold end.
 42. A method as claimed in claim 41 in which said coldend of the cylinder is both heated and cooled by means based on thePeltier effect.